Sharp-1 modulates the cellular response to DNA damage

FEBS Letters 584 (2010) 619 624 journal homepage: www.febsletters.org Sharp-1 modulates the cellular response to DNA damage Jian-Jun Liu a, Teng-Kai Chung a,b, Jiali Li b, Reshma Taneja a,b, * a Department of Physiology, National University of Singapore, Block MD9, 2 Medical Drive, Singapore 117597, Singapore b Department of Developmental and Regenerative Biology, Mount Sinai School of Medicine, New York 10029, USA article info abstract Article history: Received 13 October 2009 Revised 8 December 2009 Accepted 9 December 2009 Available online 17 December 2009 Edited by Angel Nebreda Keywords: Apoptosis DNA damage Growth arrest Dec2 DNA damage checkpoints are essential for maintenance of genome integrity. We report here that inducible overexpression of the transcription factor Sharp-1 results in an S and G2/M cell cycle arrest, concomitant with the upregulation of Brca1 and GADD45a expression. In addition, we show that endogenous Sharp-1 mrna is increased by DNA-damaging agents. Consistently, Sharp-1 overexpressing cells exhibit reduced apoptosis in response to chemotherapeutic drugs along with lower p53 expression and activity. Our studies identify a novel function for Sharp-1 in cell cycle arrest and DNA damage-induced apoptosis. Inappropriate Sharp-1 expression may therefore be associated with tumorigenesis. Ó 2009 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved. 1. Introduction Genome integrity is dependent on the activation of checkpoints that block cell cycle progression [1 5]. In response to cellular stress such as DNA damage, mammalian cells arrest in the G1, S, or G2/M phase that allows them to repair DNA. The G2 checkpoint plays a pivotal role in this process by inhibiting the entry of cells harboring DNA damage into mitosis. If the damage cannot be repaired, programmed cell death (apoptosis) is initiated to eliminate genetically altered cells from the proliferative pool. The tumor suppressor p53 plays a key role in the DNA-damage response. In addition to its established role in G1 arrest through regulation of p21 cip1/waf1 expression, p53 plays a role in G2/M arrest through regulation of GADD45a, 14-3-3, and cyclin B1 [5]. p53 also regulates the expression of genes required for apoptosis which include Puma, Noxa and Bax [6,7]. A failure of this surveillance response results in genetic alterations that lead to genome instability and development of cancers, immunodeficiency, neurodegenerative diseases, and premature ageing [1 5]. We have previously demonstrated that the transcription factor Sharp-1/Dec2 is widely expressed, and plays key roles in cellular differentiation [8 10]. The hitherto unstudied role of Sharp-1 in regulation of cellular growth arrest and apoptosis * Corresponding author. Address: Department of Physiology, National University of Singapore, Block MD9, 2 Medical Drive, Singapore 117597, Singapore. Fax: +65 6778 8161. E-mail address: phsrt@nus.edu.sg (R. Taneja). are addressed in this present report. We demonstrate that endogenous Sharp-1 mrna is up-regulated by genotoxic agents, and its overexpression results in S and G2/M cell cycle arrest. Sharp-1 upregulates the expression of Brca1 and GADD45a, that control cell cycle arrest and DNA repair [11,12]. Consistent with its ability to block cell cycle progression, Sharp-1 overexpressing cells exhibit an abrogated apoptotic response in presence of DNA damaging triggers and express reduced levels of p53 and its transcriptional targets. Together, our studies demonstrate that Sharp-1 plays a key role in protection of cells from anti-cancer drugs, and suggest that its altered function or expression may be associated with tumorigenesis and or tumor progression. 2. Materials and methods 2.1. Cell culture Doxycycline inducible Sharp-1 expressing cell lines were generated using the rtta inducible system. NIH3T3 fibroblast cells (ATCC) were transfected with EF1prtTA [13] and selected with G418. After selection, cells were co-transfected with myc-tagged Sharp-1 in puhd10-3 and a puromycin resistance vector [14]. Individual colonies were analyzed for Sharp-1 expression in response to doxycycline (2 lg/ml). Cells containing both vectors but not induced with doxycycline were used as controls. Cells were cultured in DMEM supplemented with 10% bovine serum. 0014-5793/$36.00 Ó 2009 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.febslet.2009.12.011

620 J.-J. Liu et al. / FEBS Letters 584 (2010) 619 624 2.2. Colony formation assay Colony formation assays were done as described [14]. Colonies were stained with crystal violet. The dye was extracted and the absorbance read at 570 nm. 2.3. Cell cycle analysis Cell cycle analysis was done as described [14]. Flow cytometry was performed in a BD Coulter flow cytometer and analyzed using WINMDI software. Fig. 1. Sharp-1 causes cell cycle arrest. (A) Sharp-1 expression was induced with doxycycline for 0 72 h and analyzed by Western blot using anti-myc antibody. GAPDH was used as a loading control. Cells were immunostained 24 h after doxycline treatment and analyzed by fluorescence microscopy. Nuclei were stained with DAPI (lower panel). (B) Cells were plated in triplicates at a density of 0.1 10 6 and left untreated (control) or treated with doxycycline (Sharp-1). Twenty-four hours later, cell numbers were counted by trypan blue assay. Error bars indicate mean ± standard error (S.E.). (C) NIH3T3 cells without or with doxycycline treatment were cultured at a low density for 10 days, and colonies were stained with crystal violet. The panel on the left shows representative plates from two independent experiments. For quantification, the dye was extracted and the absorbance read at 570 nm (right). Sharp-1 expressing cells showed a significant reduction in colony formation (*P < 0.05). (D) Control and cells treated with doxycycline (24 h) were analyzed by FACS. The left panel shows representative histograms of cell cycle profiles in cells without (control) and with Sharp-1 overexpression. Quantification (right panel) of the percentage cells undergoing S and G2 arrest indicated a significant increase in Sharp-1 overexpressing cells compared to controls (*P < 0.05). Results are means ± S.E. from four independent experiments.

J.-J. Liu et al. / FEBS Letters 584 (2010) 619 624 621 2.4. Caspase-3 activity Caspase-3 activity assays were based on cleavage of the substrate Ac-DEVD-pNA [Biomol International]. Cleavage of the substrate was measured spectrophotometrically at 405 nm and the activity calculated. 2.5. Reverse transcriptase PCR (RT-PCR) Semi-quantitative, and quantitative RT-PCR was done as described [10,15]. Primers sequences are listed in Supplementary Table 1. 2.6. Statistical analysis Error bars indicate means ± standard error (S.E.). Statistical analysis was performed by Student s t-test and P values < 0.05 were considered statistically significant. 3. Results and discussion 3.1. Sharp-1 causes growth arrest To investigate the role of Sharp-1 in growth arrest and apoptosis, we established stable cell lines in NIH3T3 cells that express myc-tagged Sharp-1 in a doxycycline inducible manner. Cells containing the Sharp-1 construct, but not induced with doxycycline, were used as controls. To examine the kinetics of Sharp-1 induction, cells were left untreated (time 0) or treated with doxycycline for various time points and analyzed by Western blot analysis. In the absence of doxycycline, Sharp-1 expression was not detectable using anti-myc antibody (Fig. 1A, time 0 h). Treatment of cells with doxycycline resulted in the induction of Sharp- 1 within 8 h, which was apparent until 72 h (Fig. 1A). Immunofluorescence analysis indicated that Sharp-1 was localized in the nucleus (Fig. 1A, lower panel). Compared to control cells, the induction of Sharp-1 expression resulted in a significant reduction of cell numbers (Fig. 1B). To confirm the growth inhibitory effect of Sharp-1, colony formation assays were performed. NIH3T3 cells were cultured in the absence or presence of doxycycline and colony formation was visualized by crystal violet staining (Fig. 1C, left panel). Sharp-1 expression resulted significant decrease in colony numbers compared to control cells (Fig. 1C, right panel). Together, these results demonstrate that Sharp-1 is a potent growth suppressor. To investigate whether Sharp-1 mediated growth suppression is due to an effect on the cell cycle, or on apoptosis, we first performed cell cycle analysis (Fig. 1D). Interestingly, 24 h after induction of Sharp-1, a significant increase in the percentage of cells in the S and G2/M phases was evident, with no obvious increase in the sub-g1 population (Fig. 1D, left panel). We then analyzed the mechanisms underlying growth arrest induced by Sharp-1. The G2/M checkpoint is controlled by the cyclin B/Cdc2 complex. Compared to control cells, cyclin B1 expression was somewhat increased after induction of Sharp-1, whereas the levels of p-cdc2-tyr15, which inhibits cyclin B/ Cdc2 activity [16] remained unchanged (Fig. 2A). Moreover, no changes in the levels of p21 and Cdk2 were apparent. However, p53 levels were increased 36 h after Sharp-1 expression compared to control cells. One of the key p53 target genes involved in G2/M arrest is GADD45a [11]. In control cells, GADD45a was slightly increased over time. Interestingly, GADD45a mrna was up-regulated by Sharp-1 even prior to the induction of p53, indicating that GADD45a may be directly regulated by Sharp-1 (Fig. 2B). Similarly, Brca1, which is required for S-phase arrest and DNA-repair was induced by Sharp-1 (Fig. 2B). A hallmark of entry into mitosis is the phosphorylation of histone H3 [17]. In control cells, histone H3 phosphorylation (ser 10) was increased at 16 h (Fig. 2C). In contrast, Sharp-1 overexpressing cells showed a marked reduction in phospho H3 levels confirming that Sharp-1 disrupts G2 progression. Given the S-phase arrest induced by Sharp-1, we further examined genes involved in DNA-replication or repair [1 5,12]. However, the expression of RPA, 53BP1, MDC1 and PCNA was not altered by Sharp-1 overexpression (Fig. 2D and E). 3.2. Sharp-1 is induced by genotoxic agents and protects cells from apoptosis The DNA-damage pathway activates the G2/M checkpoint, which provides a survival mechanism in mammalian cells [18]. Since Sharp-1 expression results in a pronounced G2/M arrest, we investigated its role in the DNA-damage response. We first analyzed whether Sharp-1 expression is modulated by DNA damage. NIH3T3 cells were treated with the anti-cancer drugs 5-FU, cisplatin, and etoposide (Fig. 3A and B). Interestingly, similar to GAD- D45a, Sharp-1 mrna was elevated by all genotoxic stimuli (Fig. 3A, left panel and B) suggesting that it may modulate the cellular response to chemotherapeutic drugs. Sharp-1 expression however, was not increased by prolonged culture in the absence of drug treatment (Fig. 3A, right panel). Since Sharp-1 is up-regulated by genotoxic agents and triggers arrest, we hypothesized that Sharp-1 expression may favor cell survival in face of DNA damage. We therefore examined the effect of Sharp-1 on DNA-damage-induced apoptosis. Sharp-1 was induced with doxycycline for 8 h, and cells were then treated with 5-FU or cisplatin. The extent of apoptosis was assayed by flow cytometry using the sub-g1 fraction as a surrogate marker. In control cells treated with 12.5 lm or 25lM 5-FU alone, 27% and 30% of cells underwent apoptosis, respectively. In Sharp-1 overexpressing cells however, apoptosis was significantly reduced to 16% and 18%, respectively (Fig. 3C and D). Similar effects were seen with cisplatin treatment (Supplementary Fig. 1). Consistent with the above results, Sharp-1 overexpressing cells exhibited a significant decrease in caspase-3 activity in response to 5-FU compared to control cells (Fig. 3E). Moreover, compared to control cells, Sharp-1 overexpressing cells showed a striking reduction of the mitotic marker phospho-histone H3 (Fig. 3F), confirming that the cells are arrested in the G2 phase. 3.3. Sharp-1 attenuates the p53 response to DNA damage Activation of the p53 pathway is essential for the response to DNA damage. To study the mechanisms underlying apoptosisresistance conferred by Sharp-1, we analyzed p53 expression levels and its transcriptional activity. NIH3T3 cells were pre-treated with doxycycline for 16 h to induce Sharp-1 expression. Cells were then co-treated with 5-FU for 0, 3, 8 and 24 h. Sharp-1 was expressed for the duration of drug treatment (Fig. 4A). In control cells, p53 was significantly up-regulated as early as 3 h (Fig. 4B). In contrast, the p53 response was attenuated three and 8 h after 5-FU treatment in Sharp-1 expressing cells, although at late stages there was no obvious difference. Concomitantly the expression of p21 waf1/cip1 and Bax was reduced suggesting that p53 transcriptional activity is attenuated by Sharp-1 expression. In contrast, Bcl2 levels were increased in Sharp-1 overexpressing cells. To further examine p53 activity, we analyzed the mrna levels of several p53 target genes (Fig. 4C). Consistent with reduced apoptosis and p53 levels, the expression of Puma, Noxa, Bax,

622 J.-J. Liu et al. / FEBS Letters 584 (2010) 619 624 Fig. 2. Sharp-1 induces GADD45a and Brca1 expression. (A) Cells were left untreated ( ) or treated with doxycycline (+Doxy) for 16 and 36 h. Sharp-1, cyclin B1, phosphorylated cdc2 (Tyr 15), CDK2, p21 waf/cip1 and p53 were analyzed by Western blot. (B) GADD45a and Brca1 mrna was analyzed by quantitative RT-PCR. GAPDH was used as an internal control. (C) The level of the mitotic marker H3 (ser 10) was determined before after doxycycline treatment by Western blot. (D) Expression of RPA, 53BP1, and MDC1 was analyzed by RT-PCR. The results are representative of two independent experiments. (E) PCNA and GAPDH levels were analyzed by Western blot. and Stra13/Dec1 [6,7,15] were not appropriately induced by 5-FU in Sharp-1 overexpressing cells. In contrast, GADD45a was elevated compared to control cells at early time points and not induced further. It is noteworthy that Sharp-1 expression alone is sufficient to down regulate Bax and Stra13 expression even in the absence of 5-FU, and upregulate GADD45a levels (Figs. 2 and 4; time 0, +doxycycline). On the other hand, the reduced expression of Puma and Noxa may be due to reduced p53 activity as the expression of these genes is not altered by Sharp-1 alone. We have previously reported that Stra13 is repressed by Sharp-1, and regulates DNA-damage-induced apoptosis [9,15]. Consistent with these studies Stra13 is down-regulated in Sharp-1 expressing cells that are resistant to apoptosis. Similar to the responses seen with 5-FU, Sharp-1 overexpressing cells also showed reduced levels of p53, p21 and Bax in response to cisplatin (data not shown).

J.-J. Liu et al. / FEBS Letters 584 (2010) 619 624 623 Fig. 3. Sharp-1 is up-regulated by DNA-damaging stimuli and inhibits apoptosis. (A) NIH3T3 cells were treated with 30 lm 5-FU for 0, 3, 8, 12 and 24 h. Sharp-1 and GADD45a expression was analyzed by RT-PCR. 36B4 was used as an internal control (left panel). Sharp-1 mrna was examined in cells cultured for various time points by RT- PCR (right panel). (B) NIH3T3 cells were treated with 2 lm etoposide or 20 lm cisplatin for 0, 24 and 48 h or 0, 8 and 16 h, respectively. Sharp-1 and GADD45a expression was analyzed by RT-PCR. (C) NIH3T3 cells were pre-treated with doxycycline for 8 h to induce Sharp-1, and were then co-treated with 12.5 lm or 25lM 5-FU for 72 h. Representative histograms of cell cycle profiles without and with Sharp-1 overexpression are shown. (D) The percentage of sub-g1 fraction in cells treated with 5-FU in the absence and presence of Sharp-1 overexpression was determined. Sharp-1 overexpression resulted in a significant decrease (*P < 0.05) in the percentage of apoptotic cells. Error bars indicate mean ± S.E. (E) Caspase-3 activity was determined in control and Sharp-1 expressing cells under conditions described in (C). Sharp-1 expressing cells exhibited a significant (*P < 0.05) decrease in caspase-3 activity compared to controls. (F) Control and doxycycline induced cells (16 h) were treated with 5-FU and phospho H3 (ser10) levels was determined. Taken together, our studies indicate a novel regulatory axis through which Sharp-1 regulates survival of normal cells via activation of cell cycle checkpoints. This function provides protection from the cytotoxic effects of DNA damaging agents. This survival mechanism is applicable to normal cells (this study). It is conceivable that inappropriate Sharp-1 expression plays a role tumorigen-

624 J.-J. Liu et al. / FEBS Letters 584 (2010) 619 624 Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.febslet.2009.12.011. References Fig. 4. Sharp-1 inhibits expression of pro-apoptotic p53 targets. (A and B) Cells were left untreated ( ) or induced with doxycycline (+) for 16 h prior to addition of 15 lm 5-FU for 0, 3, 8 and 24 h. Sharp-1, p53, p21, Bax, and Bcl2 were analyzed by Western blot. (C) The expression of Noxa, Puma, Bax, Stra13 and GADD45a was analyzed by RT-PCR in control and doxycycline induced cells after various time points of 5-FU treatment as indicated. esis and/or tumor progression and other disorders such as premature ageing arising from cell survival subsequent to DNA damage. Acknowledgements We are grateful to Drs. H. Bujard and P.B. Fisher for providing tetracycline inducible vectors, and Dr. R. Gopalkrishnan for comments on the manuscript. This work was supported in part by NCI, a Scholar Award from the Leukemia and Lymphoma Society, and the National University of Singapore (R.T.). [1] Bartek, J. and Lukas, J. (2007) DNA damage checkpoints: from initiation to recovery or adaptation. Curr. Opin. Cell Biol. 19, 238 245. [2] Motoyama, N. and Naka, K. (2004) DNA damage tumor suppressor genes and genomic instability. Curr. Opin. Genet Dev. 14, 11 16. [3] Zhou, B.B. and Elledge, S.J. (2000) The DNA damage response: putting checkpoints in perspective. Nature 408, 433 439. [4] Meek, D.W. (2009) Tumor suppression by p53: a role for the DNA damage response? Nat. Rev. Cancer 9, 714 723. [5] Taylor, W.R. and Stark, G.R. (2001) Regulation of the G2/M transition by p53. Oncogene 20, 1803 1815. [6] McCurrach, M.E., Connor, T.M., Knudson, C.M., Korsmeyer, S.J. and Lowe, S.W. (1997) Bax-deficiency promotes drug resistance and oncogenic transformation by attenuating p53-dependent apoptosis. Proc. Natl. Acad. Sci. USA 94, 2345 2349. [7] Villunger, A., Michalak, E.M., Coultas, L., Mullauer, F., Bock, G., Ausserlechner, M.J., et al. (2003) P53- and drug-induced apoptotic responses mediated by BH3-only proteins puma and noxa. Science 302, 1036 1038. [8] Azmi, S., Ozog, A. and Taneja, R. (2004) Sharp-1/DEC2 inhibits skeletal muscle differentiation through repression of myogenic transcription factors. J. Biol. Chem. 279, 52643 52652. [9] Azmi, S., Sun, H., Ozog, A. and Taneja, R. (2003) MSharp-1/DEC2, a basic helix loop helix protein functions as a transcriptional repressor of E box activity and Stra13 expression. J. Biol. Chem. 278, 20098 20109. [10] Gulbagci, N.T., Li, L., Ling, B., Gopinadhan, S., Walsh, M., Rossner, M., et al. (2009) SHARP1/DEC2 inhibits adipogenic differentiation by regulating the activity of C/EBP. EMBO Rep. 10, 79 86. [11] Liebermann, D.A. and Hoffman, B. (2007) Gadd45 in the response of hematopoietic cells to genotoxic stress. Blood Cells Mol. Dis. 39, 329 335. [12] Durant, S.T. and Nickoloff, J.A. (2005) Good timing in the cell cycle for precise DNA repair by BRCA1. Cell Cycle 4, 1216 1222. [13] Gopalkrishnan, R.V., Christiansen, K.A., Goldstein, N.I., DePinho, R.A. and Fisher, P.B. (1999) Use of the human EF-1alpha promoter for expression can significantly increase success in establishing stable cell lines with consistent expression: a study using the tetracycline-inducible system in human cancer cells. Nucleic Acids Res. 27, 4775 4782. [14] Sun, H. and Taneja, R. (2000) Stra13 expression is associated with growth arrest and represses transcription through histone deacetylase (HDAC)- dependent and HDAC-independent mechanisms. Proc. Natl. Acad. Sci. USA 97, 4058 4063. [15] Thin, T.H., Li, L., Chung, T.K., Sun, H. and Taneja, R. (2007) Stra13 is induced by genotoxic stress and regulates ionizing-radiation-induced apoptosis. EMBO Rep. 8, 401 407. [16] Berry, L.D. and Gould, K.L. (1996) Regulation of Cdc2 activity by phosphorylation at T14/Y15. Prog. Cell Cycle Res. 2, 99 105. [17] Prigent, C. and Dimitrov, S. (2003) Phosphorylation of serine 10 in histone H3, what for? J. Cell Sci. 116, 3677 3685. [18] O Connell, M.J., Walworth, N.C. and Carr, A.M. (2000) The G2-phase DNAdamage checkpoint. Trends Cell Biol. 10, 296 303.